FIG. 8 shows a conventional optical distance measuring apparatus. This apparatus is configured to receive reflected light of a spot light applied to an object to be measured and measure the distance to the object by trigonometry or triangulation.
Specifically, a light beam emitted from a light emitting element 101 placed on a datum point, or origin, O is converted to a generally parallel light beam by a light emitting lens 102 placed on point A(0, d), which beam impinges on an object to be measured 103 at point B(0, y) as a spot light. The light beam reflected by the object 103 is condensed by a light receiving lens 104, placed on point C(L, d), is focused onto point D(L+1, 0) on a position detecting element (e.g., PSD: Position Sensitive Detector) 106 placed on the x axis, and thereby forms a received light spot.
Given that a line passing through the point C, i.e., the center of the light receiving lens, and being parallel to the y axis intersects the x axis at point E(L, 0), a triangle ABC is homothetic to a triangle ECD. Accordingly, a distance y to the object to be measured 103 can be found through detection of a position of the received light spot by the position detecting element 106, followed by measurement of a side ED (=1) and calculation of the equation below according to the principle of the trigonometry or triangulation.
                    y        =                              L            ·            d                    l                                    (        1        )            
A PSD, a linear sensor having a plurality of photodiodes, an image sensor, or the like is used as the position detecting element 106 to detect the position of an optical center of gravity of the received light spot projected onto the position detecting element 106. In FIG. 8, reference numeral 108 represents an axis of emitted light and 109 represents an axis of received light.
However, it is only when the inter-lens distance L and the distance d between the position detecting element and the light receiving lens are fixed that the distance to the object can be accurately determined by equation (1) above. In general, in optical distance measuring apparatuses, light emitting and receiving lenses are fixed or secured by a casing made of a light blocking resin for the purpose of cost reduction.
Resins used for the casing to secure the lenses generally have a large thermal expansion coefficient. Therefore, when the ambient temperature changes, the resin casing expands or contracts, resulting in changing of the inter-lens distance L. As a result, there arises a problem that, as shown in FIG. 9, broken lines indicating the optical axes at the time of measurement are shifted from solid lines indicating the optical axes at the time of room temperature so that the position of the received light spot at the time of measurement is shifted from the position of the received light spot at the time of room temperature toward the outside, in spite that the object to be measured remains positioned at the same distance. As a result, for example, when the temperature rises, the object will be found to be at a smaller distance than it is. In FIG. 9, reference numeral 201 indicates a lead frame, 202 indicates a light emitting element, 203 indicates a light receiving element, 204 indicates a signal processing IC, 205 indicates a light permeable resin, 206 indicates a light blocking resin, 207 and 208 indicate windows, 209 indicates a light emitting lens, 210 indicates a light receiving lens, 211 indicates a casing, 212 indicates a light blocking wall, and 213 indicates an object to be measured.
In order to avoid such a problem, in an optical distance measuring apparatus shown in FIG. 10, which is disclosed in PTL 1 listed below, a light emitting system consisting of a light emitting element and a light emitting lens and a light receiving system consisting of a light receiving element and a light receiving lens are connected to each other by a flexible material. In this way, a positional relationship between the light emitting and receiving elements and the light emitting and receiving lenses are maintained even when the thermal expansion occurs such that accuracy in distance measurement is maintained.
Also, in an optical distance measuring apparatus shown in FIG. 11, which is disclosed in PTL 2 listed below, a same material is used for a holding member for light receiving elements, a holding member for lenses, and a connecting member for connecting the holding members. The entire structure is thereby allowed to be evenly expandable, so that a decrease in distance measurement accuracy due to temperature changes is prevented.
However, the solutions employed in those optical distance measuring apparatuses are intended to maintain the positional relationship among the lenses and the light emitting and receiving elements so as to satisfy the principle of trigonometry or triangulation when the entire apparatus is uniformly subject to a temperature change due to a change in ambient temperature, and not intended to maintain such positional relationship when the light emitting and/or receiving element per se produces heat due to energization of these elements.
On the other hand, an optical distance measuring apparatus shown in FIG. 12, which is disclosed in PTL 3 listed below, has a temperature sensor for measuring the temperature of the holding member for the lenses and a temperature sensor for measuring the temperature of the holding member for the light receiving elements in order to compensate for the self-heating of the light receiving elements. In this apparatus, based on output from those temperature sensors, thermal expansion of the components of the apparatus due to self-heating after the energization of the light receiving elements is detected so that the accuracy in distance measurement, or ranging accuracy, is maintained.
However, this optical distance measuring apparatus requires the temperature sensor for measuring the temperature of the holding member for the light receiving elements and the temperature sensor for measuring the temperature of the holding member for the light receiving lenses, which is a problem. These temperature sensors cannot be incorporated in the light emitting elements. Instead, they have to be individually placed in contact with the respective holding members. This arrangement makes the configuration of the apparatus complicated and also requires wiring for taking in outputs from the temperature sensors. As a result, the optical distance measuring apparatus has a comparatively complicated structure. This may lead to an increased number of assembling steps and difficulty in providing a distance measuring apparatus at low cost.
Accordingly, the present inventor made an optical distance measuring apparatus shown in FIGS. 13A and 13B, which is disclosed in PTL 4 listed below, in which the light emitting and receiving lenses are formed on a plate (of, for example, 42 alloy) and the plate is integrated with a base body including the light emitting and receiving elements and a light blocking wall, in order to improve temperature characteristics of the apparatus due to the ambient heat. Further, in this optical distance measuring apparatus, a light emitting header mounted with the light emitting element is connected with a light receiving header mounted with the light receiving element by a lead frame, as shown in FIG. 14, to suppress the expansion of a spacing or distance between the light emitting element and the light receiving element caused by self-heating of the elements and thereby improve the temperature characteristics due to self-heating.